Method and apparatus for lithography in semiconductor fabrication

ABSTRACT

A method for lithography in semiconductor fabrication is provided. The method includes placing a semiconductor wafer having a plurality of exposure fields over a wafer stage. The method further includes projecting an extreme ultraviolet (EUV) light over the semiconductor wafer. The method also includes securing the semiconductor wafer to the wafer stage by applying a first adjusted voltage to an electrode of the wafer stage while the EUV light is projected to a first group of the exposure fields of the semiconductor wafer. The first adjusted voltage is in a range from about 1.6 kV to about 3.2 kV.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No.62/584,992, filed on Nov. 13, 2017, the entirety of which isincorporated by reference herein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometric size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling-down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling-down has also increased the complexity ofprocessing and manufacturing ICs.

A lithography exposing process forms a patterned photoresist layer forvarious patterning and doping processes, such as etching or ionimplantation. In a typical lithography process, a photosensitive layer(resist) is applied to a surface of a semiconductor substrate, and animage of features defining parts of the semiconductor device is providedon the layer by exposing the layer to a pattern of radiation. Assemiconductor processes evolve to provide for smaller criticaldimensions, and devices reduce in size and increase in complexityincluding number of layers, a way of accurately patterning the featuresis in order to improve the quality, reliability, and yield of thedevices.

Although numerous improvements to the methods of performing alithography exposing process have been invented, they have not beenentirely satisfactory in all respects. Consequently, it would bedesirable to provide a solution to improve the lithographic system so asto increase the production yield of the semiconductor wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 shows a schematic and diagrammatic view of a lithographic system1, in accordance with some embodiments.

FIG. 2 shows a schematic cross-sectional view of a wafer stage inaccordance with some embodiments.

FIG. 3 shows a schematic top view of wafer stage, in accordance withsome embodiments.

FIG. 4 shows a schematic top view of wafer stage, in accordance withsome embodiments.

FIG. 5 shows a schematic top view of wafer stage, in accordance withsome embodiments.

FIG. 6 shows a flow chart illustrating a method for performing alithography exposing process, in accordance with some embodiments.

FIG. 7 shows a schematic view of one stage of a method for a lithographyexposing process by applying an initial voltage to a wafer stage, inaccordance with some embodiments.

FIG. 8 shows a schematic view of one stage of a method for a lithographyexposing process by applying an initial voltage to a wafer stage, inaccordance with some embodiments.

FIG. 9 shows a schematic view of one stage of a method for a lithographyexposing process by applying different first adjusted voltages depictedas a bar diagram, in accordance with some embodiments.

FIG. 10 shows a schematic view of one stage of a method for alithography exposing process by applying a second adjusted voltage to awafer stage, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matterprovided. Specific examples of solutions and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Furthermore, spatially relative terms, such as “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. It should be understoodthat additional operations can be provided before, during, and after themethod, and some of the operations described can be replaced oreliminated for other embodiments of the method.

The advanced lithography process, method, and materials described in thecurrent disclosure can be used in many applications, including fin-typefield effect transistors (FinFETs). For example, the fins may bepatterned to produce a relatively close spacing between features, forwhich the above disclosure is well suited. In addition, spacers used informing fins of FinFETs can be processed according to the abovedisclosure.

FIG. 1 is a schematic and diagrammatic view of a lithographic system 1,in accordance with some embodiments. In some embodiments, thelithographic system 1 includes a track apparatus 10, a load lock chamber30, an exposure apparatus 50, and a controlling apparatus 70. It isappreciated that the features described below can be replaced oreliminated in other embodiments of the lithographic system 1.

In some embodiments, the track apparatus 10 includes a cassette stationS1 and a processing station S2 (performs substrate processes such as aphotoresist coating process, a developing process) arranged in order. Insome embodiments, the cassette station S1 includes a cassette stage 11and a cassette 12. The cassette stage 11 is a holding portion that holdswafer cassettes (hereinafter referred to as cassettes) that are forexample four substrate cassettes. Each cassette 12 contains a pluralityof the semiconductor wafer 2. Each cassette 12 is placed on the cassettestage 11.

In some embodiments, the processing station S2 is located between thecassette station S1 and the load lock chamber 30. The processing stationS2 transfers the semiconductor wafer 2 between the cassette station S1and the load lock chamber 30.

In some embodiments, the processing station S2 is configured to carryout a photoresist coating process and a developing process, and theprocessing station S2 includes a transfer mechanism 13, a coatingchamber 14, a developing chamber 15 and a thermal chamber 16. Thetransfer mechanism 13 is a transferring portion that transfers thesemiconductor wafer 2 to and from the processing station S2. Thetransfer mechanism 13 may be elevated, moved leftward and rightward,moved forward and backward, and rotated around the vertical axis so asto transfer the semiconductor wafer 2 among the coating chamber 14, thedeveloping chamber 15, and the thermal chamber 16.

The thermal chamber 16 is adapted to carry out a pre-treatment and/orpost-treatment for the treatment to be performed in the developingchamber 15 and/or the coating chamber 14. For example, the thermalchamber 16 may include a heating unit (not shown in figures) adapted toheat each wafer 5 after subjected to the photoresist coating process.Alternatively, the thermal chamber 16 may include a post-exposure bakingunit (PEB, not shown in figures) adapted to heat each wafer aftersubjected to a lithography exposing process. Alternatively, the thermalchamber 16 may include a post-baking unit (POST, not shown in figures)adapted to perform a heating process to vaporize moisture on each wafer5 after the developing process.

In some embodiments, as shown in FIG. 1, one or more metrology tools 165are positioned in the thermal chamber 16. In some embodiments, themetrology tool 165 includes a thermal couple adapted for measuringtemperature of the semiconductor wafer 2 in the thermal chamber 16. Insome other embodiments, the metrology tool 165 is configured to measurethe topographic height of the semiconductor wafer 2 positioned in thethermal chamber 16. For example, the metrology tool 165 discharges gasonto a surface of the semiconductor wafer 2 via a nozzle (not shown inthe figures) and detects a variation in the pressure of the gas via anair pressure gauge (not shown in the figures). Afterwards, the metrologytool 165 provides a determination on the topology (e.g., level) of thesemiconductor wafer 2 or any portion thereof.

The metrology tool 165 may be electrically connected to the controllingunit 70. The measurements from the metrology tool 165 are transmitted tothe controlling unit 70 for determining one or more process parametersfor the exposure apparatus 50. It should be appreciated that the numberof metrology tool 165 should not be limited to the embodiments shown inFIG. 1 and may vary based upon the number of regions to be measured onthe semiconductor wafer 2 at the one time.

The load lock chamber 30 is located between the track apparatus 10 andthe exposure apparatus 50. The load lock chamber 30 is arranged betweenthe track apparatus 10 and the exposure apparatus 50. The load lockchamber 30 is configured for preserving the atmosphere within theexposure apparatus 50 by separating it from the track apparatus 10. Theload lock chamber 30 is capable of creating an atmosphere compatiblewith the exposure apparatus 50 or the track apparatus 10 depending onwhere the loaded the semiconductor wafer 2 is scheduled to be next. Thiscan be performed by altering the gas content of the load lock chamber 30by such means as adding gas or creating a vacuum, along with othersuitable means for adjusting the atmosphere in the load lock chamber 30.When the correct atmosphere has been reached, the semiconductor wafer 2can be accessed by the transfer mechanism 13 or 56.

The exposure apparatus 50 is configured to use radiation or ahigh-brightness light to expose a photoresist layer coated on thesemiconductor wafer 2. The exposure apparatus 50 may be genericallyreferred to as a scanner that is operable to perform lithographyexposing process with respective radiation source and exposure mode. Insome embodiments, the exposure apparatus 50 includes a vacuum vessel 51,a wafer stage 52, an optical measurement tool 53, an exposure tool 54and a transfer mechanism 56.

The vacuum vessel 51 preserves a vacuum environment at an ultra-highvacuum pressure. The wafer stage 52, the optical measurement tool 53 andthe exposure tool 54 are positioned in the vacuum vessel 51. The waferstage 52 is configured for supporting the semiconductor wafer 2. In someembodiments, the wafer stage 52 is positioned in the vacuum vessel 51and moveable between a measuring position (indicated by solid lines) andan exposure position (indicated by dotted lines) by a driving member,such as linear motor (not shown in figures). A radial and rotationalmovement of the wafer stage 52 can be coordinated or combined in orderto transfer, and deliver the semiconductor wafer 2.

The optical measurement tool 53 is configured for measuring thetopographic height of the semiconductor wafer 2 and providing adetermination on the topology (e.g., level) of the semiconductor wafer 2or any portion thereof. The optical measurement tool 53 is positionedover the wafer stage 52 when the wafer stage 52 is positioned in themeasuring position.

In some embodiments, the optical measurement tool 53 includes atransducer 531 and a collector 532. The transducer 531 is configured toemit a measuring signal (such as focused incident beam of radiation) tothe surface of the semiconductor wafer 2. The collector 532 isconfigured to receive the measuring signal reflected from thesemiconductor wafer 2. The optical measurement tool 53 may beelectrically connected to the controlling unit 70 (FIG. 1). Themeasurements (such as topology of the semiconductor wafer) from theoptical measurement tool 53 are transmitted to the controlling unit 70for determining one or more process parameters for the exposureapparatus 50.

The exposure tool 54 is configured to apply a radiation beam thatincludes a pattern in the beam's cross-section onto the surface of thesemiconductor wafer 2 so as to print the desired patterns over aphotoresist layer coated on the semiconductor wafer 2. The exposure tool54 is positioned over the wafer stage 52 when the wafer stage 52 ispositioned in the exposure position. The technical features of theexposure tool 54, according to some embodiments, are described below.

FIG. 2 is a schematic cross-sectional view of a wafer stage 52 with asemiconductor wafer 2 positioned thereon, in accordance with someembodiments for securing a semiconductor wafer 2. The wafer stage 52includes a dielectric body 521, a power supply 57 and a pair ofelectrodes 571. The electrodes 571 are embedded in a dielectric body211. The dielectric body 211 defines an insulator for the electrodes 571embedded therein.

In some embodiments, the electrodes 571 are coupled to the power supply57 in a bipolar arrangement. In a monopolar arrangement, the powersupply 57 applies a voltage to the electrode, e.g., 571, and causeselectrostatic charges, e.g., negative charges. Electrostatic charges ofthe opposite polarity, e.g., positive charges, accumulate in thesemiconductor wafer 2 on or near the back side by which thesemiconductor wafer 2 is supported on the wafer stage 52. A gap, such asG, may exist due to one or more properties on the back side of the wafersupported on a chuck, such as warpage of the wafer or the presence ofcontaminants, etc.

A clamping force (also referred to as gripping force or pressure) iscaused by the electrostatic attraction between the accumulated chargeshaving opposite polarities. The clamping force holds or secures thesemiconductor wafer 2 to the wafer stage 52. The clamping force dependson a variety of factors, including the size of the gap G, the thicknessd of the dielectric body 521 between the electrodes 571 and the topsurface of the wafer stage 52, and the voltage applied to the electrodes571.

The configuration of the wafer stage 52 should not be limited to theembodiments above. Some exemplary embodiments of the wafer stage aredescribed below.

FIG. 3 shows a schematic top view of wafer stage 52 a, in accordancewith some embodiments. The wafer stage 52 a includes a number ofelectrodes 573, 574, 575, 576, 577 and 578. The electrodes 573, 574,575, 576, 577 and 578 are configured in an arc form in which each twoelectrodes constitute resemble a circle which surrounds the center ofthe wafer stage 52 a. The electrodes 573, 574, 575, 576, 577 and 578 areelectrically connected to a power supply 57 and controlled independentlyby the power supply 57.

In some embodiments, the clamping force in at least one of theelectrodes 573, 574, 575, 576, 577 and 578 is controllable independentof the clamping force in at least another one of the electrodes. Forexample, at least the clamping force in the electrode 573 iscontrollable independent of the clamping force in the electrode 574. Insome embodiments, the clamping force in each of the 573, 574, 575, 576,577 and 578 is controllable independent of the clamping forces in theother electrodes 573, 574, 575, 576, 577 and 578. For another example,the electrodes 573, 574, 575, 576, 577 and 578 are all controllableindependent of each other.

In some other embodiments, as shown in FIG. 4, electrodes 579 of a waferstage 52 b are configured in an array of tiles in which each electrodeis one of the tiles in the array. In some other embodiments, as shown inFIG. 5, electrodes 580, 581 and 582 of a wafer stage 52 c are configuredin a ring form in which each of electrodes 580, 581 and 582 is one amonga plurality of concentric rings and surrounds the center C of the waferstage 52 c

Referring back to FIG. 1, the exposure tool 54 is configured to supply ahigh-brightness light to expose a photoresist layer coated on thesemiconductor wafer 2. In some embodiments, the exposure tool 54includes a high-brightness light source 541, an illuminator 542, a maskstage 543, a mask 544, and a projection optics module (or projectionoptics box (POB)) 545.

In some embodiments, the high-brightness light source 541 can be an EUVsource which generates an EUV light with a wavelength centered at about13.5 nm. However, it should be appreciated that the high-brightnesslight source 541 should not be limited to emitting EUV light. Thehigh-brightness light source 541 may include a radiation source, such asan ultraviolet (UV) source or a deep ultra-violet (DUV) source.

In some embodiments, the illuminator 542 includes various refractiveoptic components, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors in order to direct light from the high-brightness lightsource 541 onto the mask stage 543, particularly to a mask 544 securedto the mask stage 543. In the present embodiment where thehigh-brightness light source 541 generates light in the EUV wavelengthrange, reflective optics is employed.

The mask stage 543 is configured to secure the mask 544. In someembodiments, the mask stage 543 includes an electrostatic chuck(e-chuck) to secure the mask 544. In the present embodiment, the mask544 is a reflective mask and includes a multiple reflective multiplelayers (ML) deposited on a substrate so as to highly reflect the EUVlight.

The projection optics module (or projection optics box (POB)) 545 isconfigured for imaging the pattern of the mask 544 on to thesemiconductor wafer 2 secured to the wafer stage 52 of the exposureapparatus 50. In some embodiments, the POB 545 is a set of opticallenses. The optical lenses may include refractive optics (such as for aUV lithography system) or alternatively reflective optics (such as foran EUV lithography system) in various embodiments. The light directedfrom the mask 544, carrying the image of the pattern defined on themask, is collected by the POB 545. The illuminator 542 and the POB 545are collectively referred to as an optical module of the exposureapparatus 50.

In some embodiments, the exposure apparatus 50 further includes a numberof actuators (not shown in the figures) connected to the illuminator 542and the POB 545 to adjust the position of optic elements of theilluminator 542 and the POB 545. The actuators are electricallyconnected to the controlling apparatus 70. In addition, the actuator iscontrolled to drive the movement of the optic elements of theilluminator 542 and the POB 545 according the signals issued by thecontrolling apparatus 70. As a result, the focal length of the radiationscanned over the semiconductor wafer 2 can be adjusted.

In some embodiments, the controlling apparatus 70 includes aninformation handling system such as a computer, server, workstation, oranother suitable device and is electrically connected to all elements ofthe lithographic system 1 to perform one or more steps of the methodsand/or functions of the systems described herein.

For example, the controlling apparatus 70 is electrically connected tothe metrology tool 165 and the optical measurement tool 53 and receivesa signal in relation to the height of the semiconductor wafer 2 from themetrology tool 165 and the optical measurement tool 53. In addition, thecontrolling apparatus 70 is electrically connected to the wafer stage 52of the exposure apparatus 50 so as to control the movement of the waferstage 52, or alternatively to control the power supplied by the powersupply 57.

FIG. 6 is a flow chart illustrating a method 80 for performinglithography exposing process, in accordance with some embodiments. Forillustration, the flow chart will be described in conjunction with theschematic views shown in FIGS. 1, 3 and 7-10. Some of the stagesdescribed can be replaced or eliminated for different embodiments.

The method 80 includes operation 81, in which the semiconductor wafer 2is placed on a wafer stage, such as wafer stage 52 a shown in FIG. 3,wafer stage 52 b in FIG. 4, or wafer stage 52 c in FIG. 5. For brevity,operations of the method 80 are described with reference to wafer stage52. In some embodiments, the semiconductor wafer 2 is transferred fromthe thermal chamber 16 to the wafer stage 52 a via the load lock chamber30. In some embodiments, before the semiconductor wafer 2 is sent to theload lock chamber 30, the semiconductor wafer 2 is coated with aphotoresist layer (not shown in figures) in the coating chamber 14.Afterwards, the semiconductor wafer 2 is heated in the thermal chamber16.

In some embodiments, temperatures in multiple regions of thesemiconductor wafer 2 are measured in the thermal chamber 16, and thedata associated with the temperature is sent to the controlling unit 70.In some other embodiments, flatness (or level) in multiple regions ofthe semiconductor wafer 2 are measured in the thermal chamber 16, andthe data associated with the flatness is sent to the controlling unit70. In some embodiments, there is no metrology process for measuring theheight and/or temperature distributions of the semiconductor wafer 2.

The method 80 also includes operation 82, in which the electrodes 573,574, 575, 576, 577 and 578 of the wafer stage 52 a are controlled in aninitial mode so as to execute a first adjustment (or a coarseadjustment) in the topology of the semiconductor wafer 2.

In some embodiments, in the initial mode, the electrodes 573, 574, 575,576, 577 and 578 are supplied with different initial voltages V1 fromthe power supply 57, and the initial voltages V1 supplied to theelectrodes 573, 574, 575, 576, 577 and 578 are determined based on thetopology of the semiconductor wafer 2. For example, as shown in FIG. 7,the central region of the semiconductor wafer 2 is higher than the edgeregion of the semiconductor wafer 2. To improve the flatness of thesemiconductor wafer 2, the electrodes 573 and 576 are supplied withinitial voltages V1 which are higher than initial voltages V1 applied tothe electrodes 575 and 578. As a result, the central region of thesemiconductor wafer 2 is attracted by a greater electrostatic force thanthe edge region of the semiconductor wafer 2 to allow the semiconductorwafer 2 to be substantially located at the target level T1 as shown inFIG. 7. The initial voltage V1 may be in a range from about 1.6 kV toabout 3.2 kV.

In another example, as shown in FIG. 8, the central region of thesemiconductor wafer 2 is lower than the edge region of the semiconductorwafer 2. To improve the flatness of the semiconductor wafer 2, theelectrodes 575 and 578 are supplied with initial voltages V1 which arehigher than initial voltages V1 applied to the electrodes 573 and 576.As a result, the edge region of the semiconductor wafer 2 is attractedby a greater electrostatic force than the central region of thesemiconductor wafer 2 to allow the entire surface of the semiconductorwafer 2 to be substantially located at the target level T1 as shown inFIG. 8.

In some embodiments, the topology of the semiconductor wafer 2 isderived based on data associated with the height and/or temperaturedistributions of the semiconductor wafer 2 measured by the metrologytool 165 positioned in the thermal chamber 16. In some otherembodiments, the topology of the semiconductor wafer 2 is derived basedon historical data associated with warpage of the semiconductor wafer 2.The historical data may present an average flatness (or level) ofmultiple semiconductor wafers 2 which are processed under the sameprocess in one or more previously conducted process runs. Alternatively,the historical data may present an average temperature distribution ofmultiple semiconductor wafers 2 which are processed under the samerecipe in one or more process runs. The historical data may be recordedin a database and sent to the control unit 70 (FIG. 1) before thebeginning of operation 82.

However, it should be appreciated that many variations and modificationscan be made to embodiments of the disclosure. In some embodiments, inthe initial mode, the electrodes 573, 574, 575, 576, 577 and 578 aresupplied with the same initial voltage. The entire area of thesemiconductor wafer 2 is evenly attracted by an electrostatic forcegenerated by the electrodes 573, 574, 575, 576, 577 and 578 of the waferstage 52 a.

With the completion of operation 82, the semiconductor wafer 2 held onthe wafer stage 52 has an adjusted topology. As a result, the flatnessof the semiconductor wafer 2 is sufficiently improved, and warpage dueto non-uniform temperature distribution during the heating process inthe thermal chamber 16 (FIG. 1) is solved. However, in one certainembodiment, as shown in FIG. 9, the semiconductor wafer 2, secured bythe wafer stage 52, has not been adjusted to have a desired flatness forthe process (e.g., EUV lithography process) that is to be conducted inthe vacuum vessel 51. This uneven flatness may be a result of a presenceof a particle P between the semiconductor wafer 2 and the wafer stage52, a lower precision in the measurements of the height and/ortemperature distributions of the semiconductor wafer 2, or otherreasons. In order to improve the flatness of the semiconductor wafer 2,the following operations 83 and 84 are conducted.

In operation 83, a measuring process is conducted to obtain the adjustedtopology of the semiconductor wafer 2 which is secured by the waferstage 52 operated in the initial mode. In some embodiments, in themeasuring process, the wafer stage 52 is moved to a measuring positionas indicated by solid lines as shown in FIG. 1. Afterwards, as shown inFIG. 1, the transducer 531 emits a measuring signal (such as focusedincident beam of radiation) to the surface of the semiconductor wafer 2.The incident beam is reflected off the surface of the semiconductorwafer 2 and is received by the collector 532. Afterwards, the opticalmeasurement tool 53 determines the height of the wafer surface as afunction of position from the incident and reflected beams of radiation.The measurements (such as topology of the semiconductor wafer) from theoptical measurement tool 53 are transmitted to the controlling unit 70for determining the adjusted topology (or level) of the semiconductorwafer 2.

In operation 84, the electrodes 573, 574, 575, 576, 577 and 578 of thewafer stage 52 a are controlled in a first adjusted mode so as toexecute a second adjustment (or fine adjustment) in the topology of thesemiconductor wafer 2. In some embodiments, the electrodes 573, 574,575, 576, 577 and 578 are supplied with different first adjustedvoltages V2 from the power supply 57, and the first adjusted voltages V2are determined based on the measurements in the adjusted topology of thesemiconductor wafer 2 in operation 83. For example, as shown in FIG. 9,different regions of the semiconductor wafer 2 have different levels,and the electrodes 573, 574, 575, 576, 577 and 578 are supplied withelectric voltage having different voltages that are proportional to themeasured levels. As a result, the semiconductor wafer 2 is substantiallylocated at the target level T2, as shown in FIG. 9. In some embodiments,the first adjusted voltage V2 is in a range from about 1.6 kV to about3.2 kV.

However, it should be appreciated that many variations and modificationscan be made to embodiments of the disclosure. In some embodiments, inthe first adjusted mode, the electrodes 573, 574, 575, 576, 577 and 578are supplied with the same first adjusted voltage. As a result, theentire area of the semiconductor wafer 2 is evenly attracted by anelectrostatic force generated by the electrodes 573, 574, 575, 576, 577and 578 of the wafer stage 52 a.

In some embodiments, in operation 84, at least one electrode locatedbelow a region of the semiconductor wafer 2 which is lower than thetarget level T2, according to the measuring result in operation 83, issupplied with a minimum voltage V_(min) rather than being proportionallyadjusted according to the measurements in the adjusted topology. Forexample, as shown in FIG. 9, the regions corresponding to electrodes 574and 575 is lower than the target level T2, and the electrodes 574 and574 are supplied with an electric voltage (indicated by the barsillustrated with dotted lines) which is equal to the minimum voltageV_(min). The minimum voltage V_(min) may be equaled to the firstadjusted voltage V2 that is applied to the electrode 577 that is locatedon the target level T2 Since all electrodes are supplied with anelectric voltage higher than the minimum voltage V_(min), thesemiconductor wafer 2 can be securely hold by the wafer stage 52. Theminimum first adjusted voltage V_(min) may be about 1.6 kV.

In some embodiments, the back side of the semiconductor wafer 2 may becontaminated by a particle P. The particle P may result in an unevensurface of the semiconductor wafer 2. However, once the first adjustedvoltage V2 is applied to the wafer stage 52 a, the semiconductor wafer 2is attracted to move toward the wafer stage 52 a by the electrostaticforce generated by the electrodes 573, 574, 575, 576, 577 and 578 of thewafer stage 52 a. As a result, the particle P accumulated between thesemiconductor wafer 2 and the wafer stage 52 a is compressed to deformand is labeled as the particle P′. Therefore, an uneven surface of thesemiconductor wafer 2 due to a particle contamination in the back sideof the semiconductor wafer 2 is mitigated or avoided.

The method 80 also includes operation 85, in which a lithographyexposing process is performed. In some embodiments, in the lithographyexposing process, the wafer stage 52 is moved to an exposure position asindicated by dash lines shown in FIG. 1. Afterwards, the radiation beam,such as EUV light, with the calculated focal length is incident on thesemiconductor wafer 2 to expose a pattern of a circuit feature orportion thereof in the exposure field (e.g., portion of substrateexposed to radiation). Since the flatness of the semiconductor wafer 2is adjusted in operation 84, an overlay error of the circuit feature inthe lithography exposing process is reduced.

In some embodiments, as shown in FIG. 10, the semiconductor wafer 2 hasa number of exposure fields, such as exposure fields E1, E2, E3 and E4.The exposure fields E1, E2, E3 and E4 may be located at any location ofthe semiconductor wafer 2. In some embodiments, during the lithographyexposing process, the radiation beam from the exposure tool 54sequentially expose the exposure fields E1, E2, E3 and E4 so as toexpose a pattern in each of the exposure fields E1, E2, E3 and E4. Thefocal length of the radiation beam incident on the semiconductor wafer 2is defined in part by the illuminator 542 and the POB 545.

In some embodiments, a focal length error occurs after the exposure ofthe first group of exposure fields E1 and E2 due to thermal stressapplied to the illuminator 542 or the POB 545. For example, the focalpoint of the radiation beam may be shifted from an original focal pointFP1 to a new focal point FP2 due to deformation of optical lens 5451resulting from an increase of temperature after the exposure of thefirst group of exposure fields E1 and E2. If the remaining exposurefields, such as the second group of exposure fields E3 and E4, areexposed with out-of-focus radiation, the critical dimension of thefeatures in the exposure fields E3 and E4 may not be accepted.

In order to properly expose the remaining exposure fields, such as theexposure fields E3 and E4, the method 80 also includes operation 86, inwhich the electrodes 573, 574, 575, 576, 577 and 578 are controlled in asecond adjusted mode so as to compensate for a focal length error due toabnormality of the exposure tool 54 that may occur in the lithographyexposing process.

In some embodiments, in the second adjusted mode, the semiconductorwafer 2 is secured to the wafer stage 52 a (FIG. 7) by applying secondadjusted voltages V3 to the electrodes 573, 574, 575, 576, 577 and 578of the wafer stage 52 a. After the electrodes 573, 574, 575, 576, 577and 578 are supplied with the second adjusted voltages V3, thesemiconductor wafer 2 is lower to align with the new focal point FP2.

In some embodiments, the second adjusted voltages V3 are determinedbased on historical data associated with a focal length error afterexposure to the first group of exposure fields E1 and E2. The historicaldata associated with a focal length error may represent that the focalpoint of the radiation beam may be shifted from an original focal pointFP1 to a new focal point FP2 after the exposure of the first group ofexposure fields E1 and E2. The historical data may be recorded in adatabase and sent to the control unit 70 (FIG. 1) before the beginningof operation 86.

However, it should be appreciated that many variations and modificationscan be made to embodiments of the disclosure. In some embodiments, inthe second adjusted mode, the electrodes 573, 574, 575, 576, 577 and 578are supplied with the same post-stage voltage V3, such that the entirearea of the semiconductor wafer is evenly attracted by an electrostaticforce generated by the electrodes 573, 574, 575, 576, 577 and 578 of thewafer stage 52 a.

By adjusting the clamp force during the lithography exposing process,each exposure fields E1, E2, E3 and E4 can be exposed with proper focallengths. As a result, the overlay error can be controlled in anacceptable range regardless a deformation of optical lens, such asoptical lens 545, resulting from high heat of radiation beam.

It should be appreciated that, while, in the above descriptions, thewafer stage 52 a is utilized in the implementation of the method 80, itshould be noted that the wafer stages shown in other embodiments can beused to realize the same method 80. In addition, while differentvoltages V1, V2 and V3 are applied to the wafer stage during threedifferent operation modes (i.e., initial mode, first adjusted mode, andsecond adjusted mode), the wafer stage may be powered by the samevoltage in two sequential operation modes. For example, the electricvoltage applied to the wafer stage in the first adjusted mode ismaintained at the initial voltage if the result of measuring processshows a height difference of the semiconductor wafer is in an acceptablerange. In another example, the electric voltage applied to the waferstage in the second adjust mode is maintained at the first adjustedvoltage, if the offset of the focal point is in a range that can bemodified by the illuminator 542 or the POB 545.

Embodiments of methods and apparatus allow an improvement in flatness ofthe semiconductor wafer by controlling the power supplied for thegeneration of a clamping force with a proper voltage. Since thephotoresist layer on the semiconductor wafer can be exposed by suitablelithography parameter, an overlay error is reduced (overlay error isimproved 0.3 nm according to an experimental data). Therefore, theproduct yield of the semiconductor wafer manufactured by thelithographic system is improved.

In accordance with some embodiments, a method for lithography insemiconductor fabrication is provided. The method includes placing asemiconductor wafer having a plurality of exposure fields over a waferstage. The method further includes projecting an extreme ultraviolet(EUV) light over the semiconductor wafer. The method also includessecuring the semiconductor wafer to the wafer stage by applying a firstadjusted voltage to an electrode of the wafer stage while the EUV lightis projected to a first group of the exposure fields of thesemiconductor wafer. The first adjusted voltage is in a range from about1.6 kV to about 3.2 kV.

In accordance with some embodiments, a method for lithography insemiconductor fabrication is provided. The method includes placing asemiconductor wafer having a plurality of exposure fields over a waferstage. The method further includes moving the wafer stage with thesemiconductor wafer to a measuring position and performing a measuringprocess. The method also includes securing the semiconductor wafer tothe wafer stage by applying an initial voltage to an electrode of thewafer stage during the measuring process. In addition the methodincludes moving the wafer stage along with the semiconductor wafer to anexposure position and projecting radiation over the semiconductor waferto expose a first group of exposure fields. The method further includessecuring the semiconductor wafer to the wafer stage by applying a firstadjusted voltage which is different from the initial voltage to theelectrode of the wafer stage while the first group of exposure fields isexposed by the radiation.

In accordance with some embodiments, a lithography apparatus isprovided. The lithography apparatus includes a wafer stage configured tosecure a semiconductor wafer and having a plurality of electrodes. Thelithography apparatus further includes an exposure tool configured toperform an exposure process by projecting an extreme ultraviolet (EUV)light. The lithography apparatus also includes a power supplyelectrically connected to the electrodes of the wafer stage. Inaddition, the lithography apparatus includes a controlling unitconfigured to control power from the power supply to the electrodes tohave a first adjusted voltage during the exposure process for a firstgroup of exposure fields on the semiconductor wafer so as to secure thesemiconductor wafer to the wafer stage. The first adjusted voltage is ina range from about 1.6 kV to about 3.2 kV.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture,composition of matter, means, methods, and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method for lithography in semiconductorfabrication, comprising: placing a semiconductor wafer over a waferstage and performing a first adjustment in a topology of thesemiconductor wafer by applying an initial voltage to an electrode ofthe wafer stage; measuring the adjusted topology of the semiconductorwafer after the first adjustment is completed; performing a secondadjustment in the adjusted topology of the semiconductor wafer accordingto the measuring result by applying a first adjusted voltage to theelectrode of the wafer stage; and projecting an extreme ultraviolet(EUV) light over the semiconductor wafer.
 2. The method as claimed inclaim 1, wherein the measurement of the adjusted topology of thesemiconductor wafer comprises: scanning the semiconductor wafer using ameasuring signal; receiving the measuring signal reflected from thesemiconductor wafer; and creating the adjusted topology of thesemiconductor wafer according to the reflected measuring signal; whereinthe first adjusted voltage is determined based on the adjusted topology.3. The method as claimed in claim 2, wherein the wafer stage comprises aplurality of electrodes, and the method further comprises controllingthe wafer stage to apply different first adjusted voltages to theelectrodes; wherein the first adjusted voltage of each electrode isdetermined based on the adjusted topology of a corresponding region ofthe semiconductor wafer.
 4. The method as claimed in claim 2, furthercomprising: securing the semiconductor wafer to the wafer stage byapplying the initial voltage to the electrode of the wafer stage whilethe measuring signal is scanned on the semiconductor wafer; wherein theinitial voltage is different from the first adjusted voltage;
 5. Themethod as claimed in claim 4, further comprising: heating thesemiconductor wafer in a thermal chamber; transferring the semiconductorwafer from the thermal chamber to the wafer stage; wherein the initialvoltage is determined based on historical data associated with warpagecaused by temperature differences in different regions of thesemiconductor wafer after the heating of the semiconductor wafer in thethermal chamber.
 6. The method as claimed in claim 1, furthercomprising: directing an EUV light over a first group of exposure fieldsand a second group of exposure fields of the semiconductor wafer using aset of optical lenses; and applying the first adjusted voltage to theelectrode of the wafer stage while the first group of exposure fields isexposed by the EUV light; applying a second adjusted voltage to theelectrode of the wafer stage while the second group of exposure fieldsis exposed by the EUV light; wherein the second adjusted voltage isdetermined based on the historical data associated with a focal lengtherror of the optical lenses after exposure to the first group ofexposure fields.
 7. The method as claimed in claim 1, further comprisingcompressing a particle accumulated between the semiconductor wafer andthe wafer stage by the electrostatic force generated by the electrode ofthe wafer stage.
 8. The method as claimed in claim 1, wherein the entirearea of the semiconductor wafer is evenly attracted by an electrostaticforce generated by the electrode of the wafer stage.
 9. A method forlithography in semiconductor fabrication, comprising: placing asemiconductor wafer having a plurality of exposure fields over a waferstage; moving the wafer stage with the semiconductor wafer to ameasuring position and performing a measuring process; securing thesemiconductor wafer to the wafer stage by applying an initial voltage toan electrode of the wafer stage during the measuring process; moving thewafer stage along with the semiconductor wafer to an exposure positionand projecting radiation over the semiconductor wafer to expose a firstgroup of exposure fields; and securing the semiconductor wafer to thewafer stage by applying a first adjusted voltage which is different fromthe initial voltage to the electrode of the wafer stage while the firstgroup of exposure fields is exposed by the radiation.
 10. The method asclaimed in claim 9, wherein the measuring process comprises: scanningthe semiconductor wafer using a measuring signal; receiving themeasuring signal reflected from the semiconductor wafer; creating atopology of the semiconductor wafer according to the reflected measuringsignal; and determining the first adjusted voltage based on the topologyof the semiconductor wafer.
 11. The method as claimed in claim 10,further comprising: controlling the wafer stage to apply different firstadjusted voltages to a plurality of the electrodes of the wafer stage;wherein the first adjusted voltage of each electrode is determined basedon the adjusted topology of a corresponding region of the semiconductorwafer.
 12. The method as claimed in claim 9, further comprising: heatingthe semiconductor wafer in a thermal chamber; transferring thesemiconductor wafer from the thermal chamber to the wafer stage; anddetermining the initial voltage based on historical data associated withwarpage caused by temperature differences in different regions of thesemiconductor wafer after the heating of the semiconductor wafer in thethermal chamber.
 13. The method as claimed in claim 9, furthercomprising: directing the radiation over the semiconductor wafer using aset of optical lenses; securing the semiconductor wafer to the waferstage by applying a second adjusted voltage to the electrode of thewafer stage while a second group of exposure fields is exposed by theradiation; determining the second adjusted voltage based on thehistorical data associated with a focal length error that occurs afterexposure to the first group of exposure fields.
 14. The method asclaimed in claim 9, wherein the initial voltage is in a range from about1.6 kV to about 3.2 kV, and the first adjusted voltage is in a rangefrom about 1.6 kV to about 3.2 kV.
 15. The method as claimed in claim 9,further comprising compressing a particle accumulated between thesemiconductor wafer and the wafer stage by the electrostatic forcegenerated by the electrode of the wafer stage.
 16. The method as claimedin claim 9, wherein the entire area of the semiconductor wafer is evenlyattracted by an electrostatic force generated by the electrode of thewafer stage.
 17. A lithography apparatus, comprising: a wafer stageconfigured to secure a semiconductor wafer and having a plurality ofelectrodes; an exposure tool configured to perform an exposure processby projecting an extreme ultraviolet (EUV) light on the semiconductorwafer; and a controlling unit configured to control power supplied tothe electrodes to have a first adjusted voltage during the exposureprocess for a first group of exposure fields on the semiconductor waferso as to secure the semiconductor wafer to the wafer stage, wherein thefirst adjusted voltage is in a range from about 1.6 kV to about 3.2 kV.18. The lithography apparatus as claimed in claim 17, furthercomprising: an optical measurement tool configured to perform ameasuring process; wherein the controlling unit is further configured todetermine the first adjusted voltage based on a topology produced in themeasuring process.
 19. The lithography apparatus as claimed in claim 17,further comprising a thermal chamber configured to heat the wafer beforethe radiation is projected onto the semiconductor wafer; wherein thecontrolling unit is further configured to control the power from thepower supply to the electrodes to have an initial voltage before theexposure process; wherein the initial voltage is different from thefirst adjusted voltage and is determined based on historical dataassociated with warpage caused by temperature differences in differentregions of the semiconductor wafer after the heating of thesemiconductor wafer in the thermal chamber.
 20. The lithographyapparatus as claimed in claim 17, wherein the controlling unit isfurther configured to control the power from the power supply to theelectrodes to have a second adjusted voltage during the exposure processfor a second group of exposure fields on the semiconductor wafer;wherein the second adjusted voltage is different from the first adjustedvoltage and is determined based on a focal length of the exposure tool.